1. Field of the Invention
The present invention relates to a lithium secondary battery, and more particularly, solid polymer electrolyte having improved ionic conductivity and mechanical strength and a lithium secondary battery employing the same.
2. Description of the Related Art
As portable electric and electronic devices tend to pursue high performance and miniaturization, there has been increasing demand for high-performance secondary batteries, and a great attention has been paid to thus lithium secondary batteries as next generation batteries.
According to the kind of electrolyte used, lithium secondary batteries are classified into lithium ion batteries using liquid electrolyte and lithium ion polymer battery using polymer solid electrolyte. Since the lithium ion polymer battery using solid electrolyte is free of danger of leakage of an electrolytic solution and has excellent processability, it can be made a battery pack. Also, the lithium ion polymer battery is lightweight while not being bulky, and has a very small discharge rate. Due to such characteristics, lithium ion polymer batteries are safer than lithium ion batteries and can be easily manufactured into angular batteries or large-sized batteries.
The solid electrolyte of a lithium ion polymer battery include pure-solid polymer electrolyte, gel-type polymer solid electrolyte, hybrid polymer electrolyte and the like.
The pure solid polymer electrolyte produces a thin film by a solvent evaporation coating method, and examples thereof include polyether graft polyether electrolyte, polysiloxane electrolyte and the like. The ionic conductivity of such electrolyte is based on the local segmental motion of polymer.
The gel-type polymer solid electrolyte that produces a polymer host structure and a stable cell by adding an electrolytic solution to a polymer matrix, has better ionic conductivity characteristics at room temperature but poorer mechanical properties than the pure solid polymer electrolyte. In order to make up for the weakness in the mechanical property of the gel-type polymer solid electrolyte, crosslinking or thermosetting materials are generally further added in preparing electrolytes. The ionic conductivity of the polymer solid electrolyte is based on the mobility of ion species in an electrolytic solution.
Examples of the gel-type polymer solid electrolyte include an electrolyte prepared by mixing ethylene glycol with dimethacrylate and irrradiating UV rays into the mixture. This electrolyte exhibits excellent flexibility but is liable to be hardened by heat after UV irradiation, which makes further processing impossible. In the case of fabricating a battery using the polymer electrolyte, the interface resistance between an electrode and electrolyte increases, resulting in difficulty of putting into practice.
Another example of gel-type polymer solid electrolyte includes a crosslinked polyethyleneoxide electrolyte, which is prepared by crosslinking polyethyleneoxide to reduce crystallinity. As a result, the ionic conductivity of the electrolyte can be improved to the maximum of 10xe2x88x925 S/cm, which is still unsatisfactory to be used as a room-temperature type lithium secondary battery.
Another example of gel-type polymer solid electrolyte includes electrolyte using polyacrylonitrile, which is prepared by dissolving polyacrylonitrile in an electrolytic solution and making a gel with the temperature of the resultant structure further reduced. The thus obtained electrolyte has a poor mechanical strength and is not uniform in view of impregnation of an electrolytic solution and distribution of lithium salts, resulting in reduction of electrical characteristics, while exhibiting excellent ionic conductivity of 10xe2x88x923 S/cm.
Hybrid polymer electrolyte is prepared by injecting an electrolytic solution into a porous polymer matrix having fine pores of less than submicron dimension, by which the possibility of commercializing lithium ion polymer batteries has become maximized. However, the hybrid polymer electrolyte requires a large amount of organic solvent such as acetone in manufacturing a polymer matrix and a refining facility for recycling used organic solvent is necessary. Also, such a characteristic as ionic conductivity is very sensitive to the content of the electrolytic solution impregnated into the polymer matrix.
As described above, in the case of using the gel-type polymer electrolyte and the hybrid polymer electrolyte, the polymer matrix must impregnate a large amount of electrolytic solution to obtain a good ionic conductivity characteristic. However, existing polymer electrolytes are not still satisfiable in view of ionic conductivity and mechanical property.
To solve the above problems, it is a first object of the present invention to provide a composition for forming polymer electrolyte having improved ionic conductivity and mechanical property.
It is a second object of the present invention to provide a lithium secondary battery employing a polymer electrolyte formed using the composition for forming the polymer electrolyte.
To achieve the first object of the present invention, there is provided a composition for forming polymer solid electrolyte having a polymer resin, a plasticizer, a filler and a solvent, wherein the filler is synthetic zeolite having an affinity for an organic solvent or moisture.
To achieve the second object of the present invention, there is provided a lithium secondary battery having a cathode, an anode and polymer electrolyte interposed between the cathode and the anode, wherein the polymer electrolyte is formed by coating a composition for forming polymer electrolyte comprising a polymer resin, a plasticizer, a filler and a solvent, on a base and then dried the resultant structure, and the filler is synthetic zeolite having an affinity for an organic solvent or moisture.